1. Field of the Invention
The invention relates to a method for the detection of the presence of a dielectric passivation layer in an integrated circuit, and an integrated circuit having such a detector.
2. Discussion of the Related Art
In an integrated circuit, the active layers of the circuit are usually covered with at least one dielectric layer of protection called a passivation layer. Various methods may be used to make this layer.
The passivation layer has the primary function of preventing pollution from having adverse effects on the electrical characteristics of the integrated circuit. The layer also makes it possible to obtain a final surface that is relatively flat. In addition, the passivation layer protects the integrated circuit against possible attempts at fraud. The passivation layer, by its mere presence alone, prevents the observation, by electronic microscope, of the static internal signals of the integrated circuit. Moreover, this insulating layer (passivation layer) also prevents the use of electronic probe tips to set the state of internal signals.
There is a known way of using a passivation presence detector in order to generate an alarm signal, if necessary. This alarm signal may turn off the circuit or may be sent towards an intelligent element of the integrated circuit. Light detectors (diodes) have thus been used, and the capacitive coupling of interdigitated metal combs has also been used. In normal operation, when an input pulse is applied to one comb, an amplified pulse is obtained at the output of the other comb. This amplification is used as the basis of detection. If the passivation layer over the combs is removed, the amplification effect is smaller, giving an attenuated pulse at the output. The passivation presence detector perceives this change and generates an alarm.
However, the constant improvement of techniques for the removal of passivation layers makes it fairly easy to carry out a partial removal of the passivation layer without triggering any alarm, in spite of these passivation presence detectors. In the case of the interdigitated comb detector, it is also possible to short-circuit the two combs and apply an expected amplified signal at the output, this signal being obtained by simulation.
It has therefore become necessary to provide for a detector that can cover the maximum amount of surface area of the integrated circuit. Although possible, it is not practical to pursue a pure and simple increase in the number of known detection devices, as the electronic circuits necessary for detection (especially the comparators) would then take up an excessively large amount of space on the integrated circuit.
Accordingly, it is an object of the present invention to provide a method for detection of the presence of passivation that enables optimal monitoring of the entire surface of the integrated circuit. Another object of the present invention is to provide an integrated circuit comprising a particularly efficient detector of the presence of passivation. A further object of the present invention is to provide an integrated circuit comprising a detector of the presence of passivation that is difficult to simulate.